DESCRIPTION: (Applicant's Description) Recombinational repair protects against the cytotoxic effects of DNA damaging agents. However, a side effect of mitotic recombination is that it can also promote deletions and gene conversion events that contribute to tumorigenesis. Thus, it is critical to develop our understanding of the genetic and environmental factors that trigger mitotic recombination. Advanced tools are currently available for studying point mutations in mammals (e.g., the Big Blue mouse). However, an analogous high-throughput system for studying mitotic recombination in mammals is not yet available. Each of the few systems currently available for studying mitotic recombination in mice has a severe limitations, such as tissue type restrictions, requirement for high numbers of animals, and lack of sensitivity. Here, we propose to combine genetic engineering with mechanico- optical engineering to develop the technology to rapidly quantify homologous recombination events in multiple mammalian tissues types in situ. Mitotic recombination will be detected by reconstitution of expression green fluorescent protein in an engineered segment of DNA. Using high-throughput two-photon microscopy and state of the art computational analysis, it will be possible to quantify recombinant cells in diverse mouse tissues in situ in a matter of minutes per sample. This mouse system will provide an invaluable tool for identifying the genetic and environmental factors that modulate cancer-causing mitotic recombination events in mammals. Furthermore, the appearance of recombinant cells can be monitored over time in skin, thus revealing the differential susceptibility of stem cells and transition cells. Yet another important application will be in studying the effects of cancer chemotherapeutics on mitotic recombination and in determining how specific genetic traits effect cellular susceptibility chemotherapy-induced mitotic recombination. It is hoped that this line of research will ultimately aid in pharmacogenomics. The broad long term objective of this work is to develop technology that will improve our understanding of the genetic and environmental factors that cause cancer in people and improve our ability to develop better anti-cancer regimens.